1. Field of the Invention
This invention relates generally to laying underwater pipe and, more particularly, to installing underwater pipelines on the seabed and/or connecting the same to floating terminals at the surface.
2. Description of the Related Art
Floating production facilities require risers to provide fluid conduits between subsea equipment and the surface facility (or platform). The floating structure responds dynamically to the forces of the environment to which it is exposed. This means that the conduit connecting the structure and subsea equipment must be connected in a manner that accommodates relative motions. The conduit, of course, requires top and bottom interfaces to establish functional connections. At one time, flexible pipe was considered as the only feasible means of providing flowline connections from equipment on the sea bed to a floating production vessel. However, flexible pipe is very expensive, much more so than standard steel pipe. As a result, the use of steel pipe in deep water risers was developed as an alternative means of achieving the sea bed connection. This presented the practical problem of how to deploy a steel pipe string vertically from a starting point on the ocean floor to a hand-off and terminus at a floating production system; thereby leading to the development and use of J-Lay towers for this purpose.
The assembly and deployment of subsea steel pipelines from floating vessels usually employs a J-Lay tower, especially in deep water. The J-Lay tower provides a vertical, or nearly vertical, platform for welding lengths of pipe into a pipe string. The J-Lay tower tension feeds the pipe string from an anchor on the seabed. Prepared lengths of pipe, known as "pipe joints," are fed one at a time into the J-Lay tower and welded together to form the pipe string. The angle at which the assembled pipe string leaves the vessel to enter the water is controlled by the angle and azimuth of the tower mast and "stinger," which is an inverted structure pointing down into the water underneath the J-Lay tower, relative to the pipe string being deployed.
Previous J-Lay tower installations have employed vertical masts rigidly installed on pipe lay vessels. These installations utilized pipe tensioners integrated into the mast for control of the pipe string during deployment and hand-off operations. J-Lay towers have been manufactured with the ability to lower pipe straight down, or with an angle provided by a cone shaped stinger. Other towers have been installed at a fixed angle to allow a departure from the base at an approximation of the best average departure direction. Any angular departure of the pipeline from purely vertical in the first case or from the preset pipe angle in the second case would cause the pipeline to be bent around the stinger. Also, most current stingers employ a static, fixed structure that imposes either cylindrical or conical shaped excursion boundaries on the emerging pipe string.
Rigidly installed, vertical towers engender a number of problems caused by an inability to respond in real time to the dynamic forces encountered during pipe laying operations. For instance, welding and assembly operations are performed at the working floor on pipe with a substantial imposed moment. Other deficiencies include the fact that they do not allow: 1) control of the bending stress and tension within the pipe string as it is deployed in an arc to the sea bed; 2) the laying vessel to weather vane or rotate about the pipe and thereby prevent torsional wind-up of the pipe string; and 3) precise control of the pipe lay envelope. Further, current J-Lay tower designs omit any means for precise and accurate alignment of the pipe string and new pipe joints during the welding process. Current methods for deploying pipe also encounter problems not directly associated with the use of rigidly installed, vertical towers. None efficiently and/or automatically feed pipe joints into the tower or align the pipe joints with the tower center line to facilitate assembling the pipe string 12.
Current stinger designs have a problem in that the internal geometry of the stinger is usually fixed at the diameter of the pipe string being deployed. In order to allow the passage of large diameter packages integrated into the pipe string, the stinger must be dismantled or removed from around the hanging pipe string. Additionally, such stingers only act as guide conduits for the pipe and do not stabilize, i.e., control the alignment of, the pipe as it emerges from the weld floor.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.